Common rail fuel systems typically employ multiple fuel injectors to inject high pressure fuel into the combustion chambers of an engine. Each of these fuel injectors may include a nozzle assembly having a cylindrical bore with a nozzle supply passageway and a nozzle outlet. A needle check valve may be reciprocatingly disposed within the cylindrical bore and biased toward a closed position where the nozzle outlet is blocked. In response to a deliberate injection request, the needle check valve may be selectively moved to open the nozzle outlet, thereby allowing high pressure fuel to flow from the nozzle supply passageway into the combustion chamber.
For emissions reduction and increased engine performance, it is desired to decrease the volume of fuel delivered to a combustion chamber during an initial stage of a fuel injection event. One way to ensure accurate small volume delivery is to reduce the check seat diameter of the check valve. However, if the size of the check seat diameter is reduced, the check seat impact load needs to also be reduced to maintain injector integrity. Such a reduction may be accomplished by reducing the size of the biasing spring because the biasing spring is a significant contributor to the check seat impact load. It has been found, however, that reducing the size of the biasing spring requires a supplemental force to close the needle check valve.
An example of a fuel injector that includes a device providing a supplemental force for closing the needle check valve can be found in U.S. Pat. No. 7,188,788 (the '788 patent) issued to Augustin on Mar. 13, 2007. The '788 patent discloses a fuel injector having a needle check valve, which is biased toward a closed position by a biasing spring. A sleeve is disposed over a portion of the needle valve creating a metering landing, which effectively enlarges the diameter of the needle check valve at that location. The metering landing selectively overlaps a metering edge of a metering bore to define a fuel flow passage. During a fuel injection event, fuel is permitted to flow to the needle check valve creating enough pressure to counteract the force of the biasing spring. This allows the needle check valve to move to an open position. When the needle check valve moves toward the open position, the metering landing moves away from the metering edge, and the fuel flow passage is enlarged. The enlarged fuel flow passage allows fuel to flow freely from an upper surface of the metering landing to a lower surface of the metering landing and ultimately through an outlet of the fuel injector assembly. When it is desired to end fuel injection, fuel is prevented from flowing to the needle check valve, thereby decreasing the fuel pressure counteracting the force of the biasing spring. This allows the biasing spring to move the needle check valve toward a closed position and reduces the size of the fuel flow passage by moving the metering landing toward the metering edge. As the size of the fuel flow passage is reduced, the flow of fuel from the upper surface of the metering landing to the lower surface of the metering landing becomes restricted. This restricted flow produces a greater pressure above the upper surface of the metering landing than below the lower surface of the metering landing, thereby creating a force that assists the biasing spring to close the needle check valve.
Although the '788 patent discloses a fuel injector having a device that provides a supplemental force for closing the needle check valve, the fuel injector design may not adequately control the magnitude of the supplemental force. In particular, the size of the fuel flow passage varies during the closing and opening events. In addition, the design of the fuel injector may allow the needle check valve to move laterally, further varying the size of the fuel flow passage. The variable size of the fuel flow passage may produce unpredictable pressure differentials between the fuel above the upper surface of the metering landing and the fuel below the lower surface of the metering landing. Such unpredictable pressure differentials may ultimately lead to operational failures when closing the needle check valve due to excessive or insufficient supplemental forces.
The disclosed system is directed to overcoming one or more of the problems set forth above.